It is well known that hemoglobin and oxyhemoglobin have different optical absorption spectra and that this difference in absorption spectra can be used as a basis for an optical oximeter. Most of the currently available oximeters using optical methods to determine blood Oxygen saturation are based on transmission oximetry. These devices operate by transmitting light through an appendage such as a finger or an earlobe. By comparing the characteristics of the light transmitted into one side of the appendage with that detected on the opposite side, it is possible to compute oxygen concentrations. The main disadvantage of transmission oximetry is that it can only be used on portions of the body which are thin enough to allow passage of light. There has been considerable interest in recent years in the development of an oximeter which is capable of using reflected light to measure blood oxygen saturation. A reflectance oximeter would be especially useful for measuring blood oxygen saturation in portions of the patient's body which are not well suited to transmission measurements.
Various methods and apparatus for utilizing the optical properties of blood to measure blood oxygen saturation have been shown in the patent literature. Representative devices for utilizing the transmission method of oximetry have been disclosed in U.S. Pat. Nos. 4,586,513; 4,446,871; 4,407,290; 4,226,554; 4,167,331; and 3,998,550. In addition, reflectance oximetry devices and techniques are shown generally in U.S. Pat. Nos. 4,447,150; 4,086,915; and 3,825,342.
A theoretical discussion of a basis for the design of a reflectance oximeter is contained in "Theory and Development of a Transcutaneous Reflectance Oximeter System for Noninvasive Measurements of Arterial Oxygen Saturation," by Yitzhak Mendelson (Published Doctoral Dissertation), No. 8329355, University Microfilms, Ann Arbor, Mich. (1983). A theoretical discussion of the optical properties of blood is found in "Optical Scattering in Blood," by Narayanan R. Pisharoty, (Published Doctoral Dissertation), No. 7124861, University Microfilms, Ann Arbor, Mich. (1971).
Numerous other works have disclosed theoretical approaches for analyzing the behavior of light in blood and other materials. The following is a brief list of some of these references: "New Contributions to the Optics of Intensely Light-Scattering Materials, Part 1," by Paul Kubelka, Journal of the Optical Society of America, Volume 38, No. 5, May 1948; "Optical Transmission and Reflection by Blood," by R. J. Zdrojkowski and N. R. Pisharoty, IEEE Transactions on Biomedical Engineering, Vol. BME-17, No. 2, April 1970.
Generally, the pulse amplitude measured in a reflectance oximeter is less than one percent of the total reflected signal intensity. Therefore, inaccuracies in this measurement will cause the calculation of the blood oxygen saturation to be incorrect. Errors in the measurement of pulse amplitudes can occur when the levels of the waveforms have an upward or downward trend. When the average signal of the oximetry waveforms trend upward, the pulse amplitudes of the waveforms tend to be artificially small. Conversely, when the average signal of the oximetry waveforms trend downward, the pulse amplitudes of the waveforms tend to be artificially large. Prior systems tend to measure the pulse amplitudes of the waveform inaccurately when the levels of the waveforms have an upward or downward trend. As a result, the calculation of blood oxygen saturation for these pulses is incorrect. The method and apparatus of the present invention overcomes these difficulties, as described in greater detail below.